It has been proposed that a beta zeolite which has been ion-exchanged with a metal ion is used as a catalyst for purifying an automobile exhaust gas. In Patent Document 1, for example, a denitration catalyst comprising of a carrier that comprises of a beta zeolite having an SiO2/Al2O3 molar ratio of 15 to 300, which has been ion-exchanged with 0.1 to 15% by mass of Fe3+ ions, and ferric oxide supported on the carrier is described.
In Patent Document 2, it is described that a beta zeolite is made to carry Fe3+ by subjecting the beta zeolite to ion-exchange, wherein the beta zeolite has a framework structure of which the content of Si attributed to Q4 species observed by 29Si MAS NMR spectrum ranges from 35 to 47% by mass, and wherein SiO2/Al2O3 molar ratio is equal to or more than 20 but less than 100, and that the beta zeolite is brought into contact with an exhaust gas containing nitrogen oxides.
Patent Document 3 describes a method for reducing nitrogen oxides (NOx) to N2 in a flowing combustion exhaust gas, which comprises of oxidizing nitrogen monoxide to nitrogen dioxide on a transition metal/zeolite catalyst at catalyst bed temperatures below 50° C. and reducing NOx with the catalyst using a hydrocarbon (HC) reductant at catalyst bed temperatures below 150° C. Patent Literature 3 describes that examples of the transition metal include, cobalt, manganese, cerium, copper, iron, chromium and a mixture of two or more kinds of them, and the examples of the zeolite catalyst include a beta zeolite catalyst.
Patent Document 4 describes the reduction of nitrogen oxides by using a metal-doped zeolite which has been doped with a metal such as Fe, Co, Ni, Ag, Cu, V, Rh, Pd, Pt and Ir. According to this Document, it is described that an SiO2/Al2O3 ratio ranges from 20 to 40, and a commercially available beta zeolite is used as the zeolite in Examples.
Patent Document 1: PCT International Publication No. WO2006/011575
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2007-076990
Patent Document 3: Japanese Unexamined Patent Application (Translation of PCT Application), Publication No. 2010-502418
Patent Document 4: Japanese Unexamined Patent Application (Translation of PCT Application), Publication No. 2010-527877